1. Field of the Invention
This invention relates to steam turbine apparatus, and in particular, to extraction systems for extracting steam from the blade path of the steam turbine to an associated user apparatus.
2. Description of the Prior Art
As is well known, the steam turbine apparatus comprises a rotating shaft having a plurality of arrays of rotating blades thereon. Surrounding the bladed rotor structure is a suitable casing having depending therefrom a plurality of arrays of stationary blades disposed in an alternating relationship with the rotating blades mounted on the shaft. The casing confines and guides a suitable motive fluid, such as steam, through the alternating arrays of stationary and rotating blades in order to extract energy from the steam and convert it into rotational mechanical energy.
The steam turbine is usually connected within a power plant comprising a closed loop arrangement including a steam generator and condenser. The turbine shaft is connected to an electrical generator element which generates electrical power for an associated load. It has been found and is well known in the art that the power plant operates most economically if arrangements are provided for raising the temperature of the liquid condensate prior to its introduction into the steam generator element. For this purpose, suitable feedwater heaters are provided within the system. Also, other economic considerations impel the use of various other associated apparatus within the power plant in order to enhance the efficiency thereof.
These associated apparatus, especially the feedwater heaters, derive their heat source from the extraction of steam from within the turbine apparatus to be placed in a heat exchange relationship within the heater in order to increase the temperature of the feedwater. Therefore, there is provided several extraction zones within the turbine casing at various axial locations along the blade path in order to extract steam at various pressures and temperatures from the blade path.
The present system of extraction disposes an extraction orifice at the periphery of the blade path in a location such that the axis of the orifice is perpendicular to the axis of the turbine shaft. The orifice communicates with an extraction manifold which is disposed generally circumferentially about the casing radially outward of the extraction orifice. The manifold volume is radially constricted within the area of the horizontal joints along the horizontal centerline of the turbine casing. The mainfold is itself connected to a suitable extraction conduit which conducts the steam extracted from the blade path to the associated user apparatus.
Other extraction strategies presently utilized by the prior art include the simple expedient of placing an opening within the base of the turbine casing communicating with the blade path. The extraction pipe is directly connected to the opening provided and in this manner motive steam is extracted for use in the associated apparatus.
In general, each of the above-cited extraction systems generates severe problems which deleteriously affects reliability of the rotating blades and, in an interrelated manner, deleteriously affects the efficiency of the overall power plant. Both the case of the circumferential manifold having the radial constriction in the area of the horizontal centerline and the expedient which simply disposes the extraction opening in the base of the turbine result in a non-uniformity of extraction of steam from the blade path. It is known that a large percentage of the extraction flow carried by the extraction conduit is taken from the base of the cylinder and therefore through the lower half of the turbine blade path, while the remaining extraction flow is obtained from the cover portion of the blade path.
Such large non-uniform pressure extractions exposes the rotating blades to great static pressure differences between the pressure upstream of the rotating blade and the pressure downstream of the rotating blade in the cover portion and between the pressure of the fluid upstream of the rotating blade and the pressure downstream of the rotating blade in the base portion. That is to say, since the motive fluid pressure at the exit of the stage upstream of the rotating blade row from which steam is extracted is substantially uniform over the entire circumference of the blade path and since the pressure field downstream of the rotating blade row is distorted by the non-uniformity of extraction, one can easily appreciate that cyclic load is imposed upon the rotating of blades due to a disparity of downstream pressure in the cover and in the base. Such cyclic force imposition on the rotating blade results in a probability of blade unreliability and failure.
Interrelated to the problem of blade reliability generated by the disparity between cover and base pressure is the dimunition of operating efficiency both of the turbine itself and of the overall power plant in which the turbine is disposed. Within the turbine, either of present extraction modes, due to the large pressure disparity, create vortices which lead to losses which cannot be made up in the arrays downstream of the affected zone. Of course, such vortices may combine with the cyclic force variation to increase the possibility of blade failure.
Present systems generate losses within the extraction system which results in a lowering of the pressure of the fluid delivered to the associated user apparatus. Therefore, in the case of a heater, more steam is required to be extracted from the turbine system in order to meet the pressure demands imposed on the blade path by the heater. Therefore, more steam is of necessity extracted from the blade path with a concomitant reduction in turbine efficiency. Thus, the overall efficiency of the power plant is deleteriously affected. Thus, in order to eliminate harmful cyclic force variations imposed on the rotating blade an improved extraction system is required. Also, in this age of increased attention to energy generation, an increase in efficiency in overall electrical generation systems is imperative. It is therefore incumbent that efficiency of the overall power plant be increased by the expedient of increasing efficiency of each constituent part of the plant. Thus, from the standpoint of energy conservation, it is imperative that a more efficient extraction system be provided.